Dynamic Spectrum Access and Meteor Burst Communications

2010 paper on Dynamic Spectrum Access, considering as a existing DSA-like service Meteor Burst Communications that is based on use of meteor burst trails that are stochastic in time and space, as are terrestrial spectrum "holes" that DSA may use. This paper also explains that to date little is known of these holes, and substantial surveys and study is needed. Also discussed is use of active coordination between primary and secondary users to improve DSA. SkyTel adds comments on several other means to improve DSA: wide-area prediction of the "holes" vs. availability, and use of ad hoc mobile mesh nets where the greater net performance is in part distributed to each node to lessen breaks in service and increase data rate.

2010 paper on Dynamic Spectrum Access, considering as a existing DSA-like service Meteor Burst Communications that is based on use of meteor burst trails that are stochastic in time and space, as are terrestrial spectrum "holes" that DSA may use. This paper also explains that to date little is known of these holes, and substantial surveys and study is needed. Also discussed is use of active coordination between primary and secondary users to improve DSA. SkyTel adds comments on several other means to improve DSA: wide-area prediction of the "holes" vs. availability, and use of ad hoc mobile mesh nets where the greater net performance is in part distributed to each node to lessen breaks in service and increase data rate.

matters in the analysis of dynamicspectrum access (DSA). With some exceptions, DSAresearch has focussed on either general systems problems or problems that are analyzed largely from the perspective of theprimary user. This is understandable because one must firsthave unused radio bands if one is to have DSA. But just asessentially, DSA requires secondary users. That is not to saythat the secondary users have been ignored entirely, however.Weiss and Lehr [2] consider different strategies for DSA thatexplicitly consider secondary users. Akyildiz et.al. [3]consider user requirements as part of the spectrum analysisfunction, but does not elaborate. Chapin and Lehr [4] alsoconsider user application factors, but again in general terms.Thi paper combines the spatio-temporal analysis of [1] withthe generalized secondary user perspective adopted in [5] todetermine feasible use cases for different kinds of spectrumholes.To consider a secondary user

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application requirements inmore detail, it is important to consider both the spatial andtemporal dimensions of spectrum holes [1, 6]. Figure 1illustrates the temporal characteristics at a single point inspace

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Manuscript received November 15, 2010.Martin BH Weiss is with the School of Information Sciences, University of Pittsburgh, Pittsburgh, PA 15260 USA.

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From the WINCOM lab at Illinois Institute of Technology(http://www.cs.iit.edu/~wincomweb/24hrtv.html)

sample of any 24 hour period, one can see temporal spectrumholes (blue areas in the figure) with different characteristics.Figure 2 illustrates the spatial characteristics of spectrum [7],though it is a

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coverage plot rather than a measuredone and assumes a static signal. Here, the white areas wouldconsist of spatial spectrum holes. As with Figure 1, onemight infer spectrum holes with a variety of spatialcharacteristics.

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create a spatio-temporal taxonomy of spectrumholes by classifying their characteristics as static, periodic or stochastic in both time and space. Figure 3 combines thesedimensions and provides some examples of representativereal-world systems.This paper follows a similar approach in that it examineshow the spatio-temporal characteristics of spectrum holesaffects the QoS experienced by potential secondary users.Tonmukayakul and Weiss [5] consider end user QoS in their paper, but the purpose of that paper was to study theconditions under which secondary use might occur (from thepoint of view of potential secondary users) rather thanconsidering how the spatio-temporal characteristics of spectrum holes affect QoS. Since they were considering onlycooperative secondary sharing, they could reasonably assumethat the spectrum hole would be adequate as a result of thebargaining between primary and secondary users.

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Temporal Spectrum Holes and theSecondary User

Martin B.H. Weiss,

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SkyTel note. See SkyTelMeteor Burst Communications("MBC) Folder on Scribd, for background on planned new-generation MBC,including use of wideband radios in 30-50 MHz using Cognitive Radio/ Dnamic Spectrum Access ("CR"-"DSA"). In addition, on of the bandsSkyTel plans to use for mobile ad hoc mesh nets linked to the MBC system end nodes, will use licensed, fairly high power (up to 30 W ERP)wideband (5 MHz+ wide)M-LMSspectrum in902-928 MHzrange, which will involve the presense of secondary use by Part 15...... unlicensed radios and thus should alsouse CR-DSA for most effective performanceand least disruption to secondary uses.Thus, low band VHF forMBCand 900 MHzM-LMSare especially suitable for CR-DSA.This paper discussed why MBC is by nature aDSA service. The frequency and re-radiation/ reflective power of meteror burst trails thatallow a communication links between masterstations and remotes are stochastic, as are thespectrum "holes" in time and space interrestrial wireless availble for DSA exploitationand optimization.

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[1]

Modelling the spatial aspects of DSA-based systems has notrecieved much attention from the research community thus far,even though it is generally recognized as a characteristic of spectrum holes [6]. Some researchers have made progress inaddressing this gap in the research literature, however. Inparticular, [8] considers the spatial power spectral densityusing spatial statistics and [9] considers the spatial distributionof nodes in a communications system. Further, [10] begins tomatch the needs of primary and secondary users. Together,these contribute to an understanding of the spatial aspects of DSA as they apply to communications systems.II.

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We begin by assuming that secondary users have acommunications requirement that they seek to satisfy via awireless channel. Paraphrasing [5], these users can choose anunlicensed band, secondary use, or commercial services. Wefurther assume that these choices are ordered by net cost(which is the value of the communication less the cost of executing it). These choices also have potential qualitytradeoffs; with unlicensed bands ordered by the variability of QoS (highest to lowest). A secondary user therefore mustchoose the best QoS for the net cost.It is useful to be slightly more specific about QoS, since itdepends on the nature of the communications requirement.Communications engineers have classified user communications requirements in several ways. One is to

characterize them as “elastic” if they are tolerant of delay anddelay variation and “inelastic” if

they are not [11]. Thisrelatively gross characterization is useful to an extent.However, additional parameters, such as absolute delay, mayalso be necessary:

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What is the absolute value of end-to-end delayrequirement? (

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, the ITU-T recommends round tripdelays less than 250msec for telephone calls)

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What is the average throughput and the peak throughput?

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What delay jitter is tolerable?

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Is the communications interactive or in a broadcast mode?To simplify matters, the discussion below assumes that thethroughput achievable in a spectrum hole is affected only bythe characteristics of the spectrum hole and the bandwidthavailable. In practice, the throughput and properties of theavailable communications channel is also affected by thecompeting users, the MAC protocol used to resolve contentionand the upper layer protocols used.We further posit that, for a secondary user, communicationstakes place across a collection of nodes that are separated inspace. For successful communications, a spectrum hole mustcoincide both spatially and temporally with thecommunicating nodes for the period of the communication. If interference with the licensed user is to be avoided, theradiated signal energy of the secondary user must besubstantially contained within the spectrum hole. Thus, theantenna directionality and the location of the radiating nodesmake a difference in how a spectrum hole might be used. If asecondary user knew the contours of the spectrum hole, itsutility could be maximized through careful system design.However, building a representation of the contours of aspectrum hole would require specific context acquisitionapproaches (see, e.g., [12]).We will organize the remaining discussion according to thetemporal characteristics of spectrum holes.III.

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The simplest kind of spectrum hole to address is one that isstatic in time. The main questions that must be addressed bythe secondary user for these kinds of spectrum holes iswhether the available bandwidth is sufficient for thecommunications need and whether the spatial configuration of the nodes can be contained with the contours of the spectrumhole. It is possible to have two kinds of (spatial) spectrumholes, contiguous and non-contiguous. The distinctionbetween the two can only be addressed in the context of aparticular spatial arrangement of communications nodes.

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In many senses, this represents the easiest kind of spectrumhole to use. The main questions that communicationsengineers have to address with this configuration is:

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Is the available bandwidth sufficient to meet thethroughput requirement?

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Are all nodes that must communicate within theboundaries of the spectrum hole?If the answers to the above are both affirmative, the spectrumhole can be used for the communication requirement.The investment that will be required for such a systemdepends on (1) the required modulation efficiency and (2)whether more sophisticated antennas are required to ensure

that the secondary user’s signal energy is contained within the

spectrum hole. A figure of merit for modulation schemes isbits/Hz. Modulation schemes that offer more bits/Hz are moreefficient and, as a rule, more costly than those that offer lessefficiency. If a node is located near the edge of the spectrumhole, and if the communication is bi-directional, then it maybe necessary to use a directional antenna that is generallymore costly than an omnidirectional one.

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The practicality of this kind of spectrum hole dependswhether transmission paths exist between the spectrum holesin the non-contiguous spatial matrix.For example, the white regions in the bottom right quadrantof Figure 2 are the result of a shadowing from a series of geological ridges to the west of the town of Kiowa, Colorado.In these holes, only nodes placed

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each hole would beable to communicate via the 162-174 MHz band modelled inthe figure; communications

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spectrum holes could notbe achieved in this frequency band. Communications

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holes would have to occur out-of-band, possibly at additionalcost

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. The same would be true for non-contiguous spectrumholes that are separated by, say low power transmitters, sincetransmitting through the occupied spectrum at the operatingfrequency could result in interference with licensed users.Thus, the utility of this spectrum hole would be limited to aparticular spatial configuration; secondary users requireing aless limited one that exceeds the boundaries of the spectrumhole would incur a higher cost. .Since the spectrum hole is static in time, it would bepossible to support elastic and inelastic applications. Anyapplications would be limited in throughput, of course, by thebandwidth of the spectrum hole

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Periodic spectrum holes with different origins (andtherefore different characteristics) exist. In [1], Weiss

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Out-of-band communications channels may occur via a spectrum hole ata different frequency that is not affected by the boundary mechanism; for example, in the situation illustrated in Figure 2, a spectrum hole at a lower frequency may be able to overcome the geographic barrier that creates theboundary between the holes. Additional cost would be incurred if, for example, a commercial service would have to be purchased to maintain theneeded connectivity.

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The survey that resulted in Figure 2 indicates that the nominal bandwidthof the spectrum hole is 14MHz. If a wideband measurement apparatus wereplaced in the spectrum hole, the available bandwidth could be larger.

differentiated between

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spectrumholes, where the latter are those with a period shorter than thecontext acquisition time; fast periodic spectrum holes mightoccur in TDMA systems. Fast periodic spectrum holes canonly be discovered through cooperation with the primary user;thus, they would be well defined and users could align their communications requirements with them explicitly.Consequently, for the purposes of this paper, periodicspectrum holes are those that can be sensed and used.Rotating antenna radar is an example of this kind of spectrum hole. In this kind of system, the spectrum holechanges jointly in time and space but in a very predictableway. When the researchers in [13] measured a TerminalDoppler Weather Radar (TDWR) signal at a fixed point inspace over time, the periodicity of the radar signal becameapparent (20sec) as shown in see Figure 4

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. Also apparentwas the signal energy in the side lobes.Another example of periodic time spectrum hole can beinferred from Figure 1. In this figure, the transmitters inseveral bands are off for distinct times of day (for example,the 614-620MHz band between approximately 0100 and0500). If this temporal spectrum trace repeats daily, then theresulting spectrum hole would have contiguous space if allcommunications nodes were located in the area covered bythat television channel.From a communications perspective the questions are: Whatis the minimum delay that could be expected? What averagethroughput can be expected?

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In the context of Figure 4, if the receivers can tolerateinterference of -50dBm, then approximately 20% of therotation is taken up by the main lobe when the antennaelevation is low

rotations, the side lobe energy is sufficientlyhigh to restrict the usable bandwidth to approximately 40% of the 20 second rotational period.In summary, for the entire volume 360 second scan picturedin Figure 5, approximately 321 seconds (or 89%) would beusable (this decreases if the interference tolerance of thereceivers is lower). In the worst case, the latency would beapproximately 8 seconds due to main- and side-lobeinterference. This would prohibit inelastic communications,removing that application as a reasonable use case for thiskind of spectrum hole.

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The decreasing peak signal strength in the figure occurs because theelevation of the radar changes over time. Thus, the fixed measurementapparatus would increasingly fall outside of the main lobe of the radar beam.

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These figures are estimates derived from careful visual inspection of Figure 5.